Such a photoelectric measurement head is frequently used for determining a proportional change in a profiled section in front of a calibrating device, directly after the profiled section has left the extruder nozzle. Other uses of this measurement head in the region between extruder nozzle and calibrating device are conceivable, for instance the monitoring of the collection of dust. Such a measurement head is described in Federal Republic of Germany No. OS 32 34 126 and European Patent No. OS 0 164 029.
In the vicinity of the outlet opening of the extruder nozzle, temperatures between about 160.degree. C. and 300.degree. C. generally prevail, while in the region of the inlet openings to the calibrating device temperatures of between about +15.degree. C. and +30.degree. C. and, in the case of a few types of plastics, up to +80.degree. C. customarily prevail.
Furthermore the distance between the outlet opening of the extruder outlet nozzle and the inlet opening to the calibrating device is very slight, so that the measurement-head body must be designed with due regard of the high temperatures in the region of the outlet opening of the extruder nozzle, i.e. it must consist of suitable temperature-resistant materials. For this reason the light-transmitting coverings necessarily consist of glass.
In actual practice, problems frequently arise with measurement heads of this type since the outer surfaces of the glass coverings tend toward fogging. Further examinations in connection with measurement errors of the measurement head which surprisingly occur have shown that additives such as plasticizer and lubricants which are added to the material of the plastic profiled section and which tend to vaporize at the prevailing temperatures will deposit on the surfaces of the glass coverings. These deposits on the surfaces of the coverings affect the passage of light so that the measurement results of the measurement head become erroneous.
The object of the present invention is so to develop a photoelectric measurement head of the aforementioned type that the problem of the precipitation of volatile additives to the material of the plastic profiled section on the glass coverings is avoided in a simple manner.